Whether or not a Mauser was found in the TSBD is irrelevant, as the bullet found at Parkland after falling out of the Governor's thigh, as well as the two large bullet fragments recovered from the limousine were determined to have been fired through the barrel of the Mannlicher-Carcano rifle to the exclusion of all other rifles in the world.
No bullets or cartridges of 7.65 mm were ever recovered at the assassination scenes: the TSBD, Dealey Plaza etc.
Lead fragments recovered from the victims' bodies and the limousine were analyzed using Neutron Activation Analyses and it turned out that they all originated from the recovered bullet and fragments.
The evidence tying Oswald to the "assassination rifle" is questionable or so I am lead to believe.